Traditional IC sockets are generally constructed of an injection molded plastic insulator housing which has stamped and formed copper alloy contact members stitched or inserted into positions within the housing that are shaped to accept and retain the contact members. The assembled socket body is then generally processed through a reflow oven which melts solder balls and attaches them to the base of the contact member.
During final assembly onto the PCA, the target interconnect positions on the circuit board are printed with solder paste or flux and the socket assembly is placed such that the solder balls on the socket contacts land onto the target pads on the PCB. The assembly is then reflowed and the solder balls on the socket melt and when cooled they essentially weld the socket contacts to the PCB, creating the electrical path for signal and power interaction with the system.
During use, this assembled socket receives the packaged integrated circuits and connects each terminal on the package to the corresponding terminal on the PCB. The terminals on the package are held against the contact members by applying a load to the package, which is expected to maintain intimate contact and reliable circuit connection throughout the life of the system, without a permanent connection such that the package can be removed or replaced without the need for reflowing solder connections.
These types of sockets and interconnects have been produced in high volume for many years. As systems advance to next generation architectures, these traditional have reached mechanical and electrical limitations that mandate alternate methods.
As processors and systems have evolved, several factors have impacted the design of traditional sockets. Increased terminal counts, reductions in the distance between the contacts known as terminal pitch, and signal integrity have been main drivers that impact the socket and contact design. As terminal counts go up, the IC package essentially gets larger due to the additional space needed for the terminals. As the package grows larger, costs go up and the relative flatness of the package and corresponding PCB require compliance between the contact and the terminal pad to accommodate the topography differences and maintain reliable connection.
The package producers tend to drive the terminal pitch smaller so they can reduce the size of the package as well as the flatness effects. As the terminal pitch is reduced, the available area to place a contact is also reduced, which limits the space available to locate a spring or contact member which can deflect without touching an adjacent contact. In order to maximize the length of the spring so that it can deflect the proper amount without damage, the thickness of the insulating walls within the plastic housing is reduced which increases the difficulty of molding as well as the latent stress in the molded housing which causes warping applied during solder reflow.
For mechanical reasons, the contacts tend to be long in order to obtain proper spring properties. Long contact members, however, tend to reduce the electrical performance of the connection by creating a parasitic effect that impacts the signal as it travels through the contact. Other effects such as contact resistance impact the self-heating effects as current passes through power delivering contacts, and the small space between contacts can cause distortion as a nearby contact influences the neighbor which is known as cross talk.
Traditional socket methods are able to meet the mechanical compliance requirements of today's needs, but they have reached an electrical performance limit. Next generation systems will operate above 5 GHz and beyond and the existing interconnects will not achieve acceptable performance levels without significant revision.